Much ancient glass was not clear and colourless. Often it was opaque and deeply pigmented to simulate coloured stones such as lapis lazuli and turquoise. From the Renaissance onwards, a distinctive kind of glass was made which resembled variegated stones such as agate or chalcedony.

While the history of glassmaking is, in part, a quest to produce a clear and colourless material that could serve, initially at least, as a substitute for natural rock crystal, it is also true that glassmakers took pains to devise recipes for clear glass of a striking hue. In the mid-eighteenth century, 'Bristol' blue glass was widely produced and popular. The development of the somewhat more rare ruby-red glass was associated closely, for a time, with the secretive and mysterious exploits of alchemists.

Colour in glass is achieved by adding metallic oxides to the main batch of ingredients that traditionally would include silica (in the form of sand or crushed pebbles), soda and lime.